Multiplex facsimile system



Patented Apr. 15, l1941 MULTIPLEX FACSIMILE SYSTEM Clarence W. Hansell, Port Jelersom'N. Y., assignor to lRadio Corporation of America, a corporation of Delaware Original application December 11, 1934, Serial No.

Divided and this application October 29, 193s, serial No. 237,628

11 Claims.

The present invention relates to improvements in multiplex facsimile printer systems, and is a division of my application Serial No, 756,926, filed December 11, 1934, now United States Patent No. 2,143,875, granted January 17, 1939.

One of the objects of the present invention is to provide a multiplex system for transmitting and receiving ,a facsimile reproduction of a single line of type, wherein there is reduced the possibility of unintelligible or incorrect reproduction of the facsimile message.

A further object is to provide a secret multiplex system which will make it extremely diicult, if not impossible, for unauthorized listeners to decipher the transmitted message, without having knowledge of the precise keying arrangement of the transmitting contacts.

A feature of the invention lies in the use of a cam arrangement for periodically changing the amount of stagger of the transmitting contacts to insure secrecy of transmission of the facsimile message.

In general, the present invention provides a multiplex facsimile system in which a single line of type can be transmitted and recorded by a single passage of the transmitter and receiver tapes through the respective channel equipments. In this way, a single line of type can be reproduced continuously over a radio circuit. The system of the invention employs a plurality of brushes or contacts, preferably ten, which are arranged in a single line to scan the message which is printed on a tape having a non-conductive backing. This tape has printed on its surface the message characters in electrically conductive letters, and the entire width of the line is arranged to be explored by the ten brushes as the tape is passed underneath them. Each brush has individually connected to it a source of oscillations of a particular frequency, preferably an audio frequency, whereby the current through each brush is varied in dependence upon contacting the electrically conductive letters. The currents through the ten brushes, which may be ten different audio frequencies required for transmission, or direct currents which control ten audio frequency currents, are then impressed on a common line circuit and closed through a transmitter for transmission over the ether, if a radio circuit is used, 'or over a line to a distant receiver if a land line circuit is used.

In order to reduce undesired fading due to the effect of carrier amplitude iluctuations, and to reduce harmonic distortions which cause interference between channels, it is proposed to elim i inate the carrier at the transmitter and to send out both side bands, and at the receiver to reintroduce the carrier before detecting the signal Waves.

In order to render the facsimile method of transmission of the invention relatively free of errors, even when letters are mutilated, the brushes or contacts of the pick-up and recording points are staggered; that is, arranged at a slant or lengthwise of the tape, so that the probability of whole letters being missed is greatly reduced. Thus, for a letter to be missed entirely, a fade or interference must last for the time required to pass several letters through the equipment.

In receiving, the individual audio frequencies corresponding to each channel are separated by means of filters, and the two rectined currents corresponding to each channel combine to reproduce the message at a scanning head, which may, if desired, be identical with that used at the transmitter. Thus both side bands may be used without introducing distortion due to phase fluctuations, and benefit is obtained from the amplitude diversity effect of using two side bands on each channel.

To make the system relatively secret, there may be provided like cams at the transmitter and receiver, synchronously operated, to continuously vary the relative positions of the transmitting and receiving brushes or contacts. Similarly, more secrecy may be achieved by frequently changing the assignments of the: radio channels among the scanning brushes or contacts,

Advantages of the invention, among others, are:

(a) By using the multiplex principle, the keying speed on each channel is reduced to a. small fraction of that required to scan a given area in a given time by the single channel method. If

ten channels are employed, then the keying speed,

or modulating frequency, of each channel is reduced to 10% of that required when a single channel is used. For this reason, by the use of suitable equipment, the echo phenomena caused by multipath transmission in the radio circuit has less distorting eflect upon the reproduction. In

'addition the reduced speed of each channel permits the use of relatively simple transmitting and receiving equipment. At the transmitter, for example, there may be used the new photoeells manufactured by the Weston Electrical Instrument Company to operate relays and control audio channels directly without requiring any vacuum tubes or battery supply. At the receiver, ordinary clectromagnetically operated vibrators circuits.

(b) By staggering the scanning brushes or contacts, it is unnecessary to have all the channels modulating at the same time, thus permitting the use of more modulation range per channel and more effective power. If it is assumed that the power of the transmitter, or its full modulation swing, used on one channel is 100%, then if ten channels are multiplexed on it the amplitude of the modulation allowable on each channel for ideal conditions is 10%, and the power used per channel is only 1%. In other words, multiplexing theoretically reduces the power per channel in proportion to the square of the number of channels. Actually, with the multiplex facsimile scheme, considerable allowance may be made for the diversity in phase of the audio modulating currents and the timing of the marking intervals on the channels. That is, the system may be so set up that all the channels never mark, and all audio frequency currents are never at their peak value with like polarity at the same time, so that one might have a considerable increase in the amount of modulation per channel due to the fact that the modulations do not all add together in phase at the same time, and very seldom, if ever, would all channels be modulated at once. By using the equipment without any particular regard to the phase relations of the ten tones, the amplitude per channel can be set at one-quarter the amplitude which could be allowed for a single tone. This corresponds to a power per channel of 6.25% instead of 1% of that allowable when all the modulation is concentrated on a single tone. Since in addition to time diversity in the channels there may be tolerated occasional slight over-modulation, the power per channel may be increased to 10% without causing undue diniculties. Since there are ten channels, the sum total of modulation therefore may amount to 100% of that obtainable on a single channel. In other words, the power efficiency of the multiplex system may be set up to be approximately the same as the efficiency of a single modulated system.

(c) The use of phase or frequency modulation rather than ampitude modulation enables obtaining ve or six times the power obtainable with amplitude modulation with an equipment of a given construction cost. Consequently, the overall effectiveness of the phase or frequency modulated multiplex facsimile printer transmitter in accordance with the present invention is approximately six times that of a single tone 'amplitude modulated transmitter.

' modulated waves.

In Fig. 1 there is shown a multiplex facsimile transmitter system for ten channels. A multitone generator l of any well known form is arranged to generate ten different audio frequencies f1, f2, f3, etc., one for each channel, which frequencies are individually passed by phase adjustors P sending filters 2, to scanwill record as rapidly as will be needed for mostv lning contacts 3, which may be brushes or needles arranged to bear down ona tape 4.

Brushes or contacts 3 are staggered, that is, arranged lengthwise of the passage of the tape to cover several letters or the major portion of a single letter. -By spreading out the contacts in this way, instead of arranging them transversely of the tape in a single line, as instanced by the prior art, I prevent the loss in reception of whole letters due to fading and noise. If desired, a synchronously operated cam, such as Il, may be used at both the transmitter and receiver to change the degree of stagger periodically to insure secrecy. This cam can not only change the degree of stagger but also even reverse its direction.

Tape 4 is comprised of a non-conductive fine tissue having written or printed thereon the message in conductive letters, this tape being passed underneath the scanning contacts. Each channel is shown as having a separate contact 3' associated with its filter and another contact. such as 3, which is in a common circuit with the other channels, whereby when the two contacts of each channel are connected together upon engaging a letter on the tape, an obvious circuit will be closed to send out the audio frequency characteristic of the channel over line 5, which line is common to all the channels. Tape 4 may be made of paper upon which is placed any suitable conductive solution, for example, potassium, ferricyanide, and sodium chloride. For that matter, the surface may be rendered conductive by marking with an ordinary lead pencil. The tape may consist of a thin foil mounted upon a stout paper backing and, if so, the surface of the foil should be covered with a nonconductive fine tissue and the message placed upon this tissue. In such case a special solution of graphite may be used for rendering the tape electrically conductive where marked by the stylus or pen. When the metal foil is used, the foil may serve as a common contact for al1 the scanning points.

Instead of using paper or a metal foil, an endless metal tape may be employed. This tape may be passed through a liquid solution, which, when dried, leaves an insulating film on the tape. Then, when writing or typewritten matter is put on the film by marking with an electrically conductive element such as graphite, in the same liquid as the insulator, the insulating material will be pushed and washed aside and its place taken by conducting material. The tape may then be run under the electrical contacts, its message sent, and subsequently it may be washed, returned to the insulating solution, and the whole process repeated.

It is preferred that the metal tape be of a.

color which is as different as possible from the marking material so as to make the record readily visible to the operator. For economical reasons, however, a polished steel tape is desired. The solution for the insulating and marking material should preferably be in water and slightly alkaline to prevent corrosion of the tape. The

IIUI

tion which reacts chemically with the material in or on the paper to deposit or precipitate out a conducting material. If desired, solutions of deliquescent material, for example a solution of deliquescent salts which retain moisture and are conducting, might be used for passage under the transmitter contacts. In the latter case, coloring matter would be added to the solution in order to make it legible on the transmitting tape. As an alternative to coloring the salt solution, substances might be Vput in which would decompose due to the passage of electrical current to form a visible record. The record would then be developed by the current iiowing through it as it passed under the transmitting contact points. The record so formed would thus rep- .resent a true copy of the message as sent through the transmitter and so would be of value as a check on the functioning of the transmitter. This may be accomplished, for example, by using iron scanning electrodes and a marking fluid containing potassium iodide or a combination of potassium ferricyanide and sodium chloride.

The tape may be impressed with the message characters eitherl by writing or by atypewriter whose keys hit upon a ribbon placed above the tape and the thin tape then passed under the sending contacts. If desired, the tape may be wound around a drum 6, as shown in Fig. 2, which drum is rotated to move the type past the scanning contacts. In using an arrangement such as shown in Fig. 2, it is preferred to use metal tape of the type mentioned above, whereupon it is only required that there be one scanning contact 3' associated with each channel, the other contact 3 in common with all of the channels being in contact with the tape through the drum, whereby closure of both contacts will occur when 3' engages a conductive symbol or letter on the tape.

As the tape passes beneath the sending contacts, there is produced a complex wave representing the sum of the waves of frequencies f1, f2, f3, etc., at any instant, depending upon the number of channels closed through by the configuration of the letter at that instant, which complex wave is transferred over line 5 through a suitable audio amplification means such as "l, and then through a land line 8 leading from the transmitter to the receiver in the case of wire transmission, or to a radio transmitter in the case of radio transmission.

The relative phase timing and diversity of the component signal currents may be adjusted by phase adjustors P so that their combined peak value is reduced in proportion to the strength of each current and the strength of all currents may be increased without over-modulating the transmitter.

At the radio transmitting station there is a source of. carrier frequency S which preferably is controlled by a crystal I0. The carrier energy is supplied symmetrically to a push-pull modulator il, as shown in Fig. 1 of the drawing, in consequence of which the carrier is eliminated from the output l2, and only the two side bands and the original modulating frequencies are produced, which are then amplified by power amplier I4 and radiated by means of an antenna I5. When desired, a filter I3 may be applied as a high pass filter which cuts off at the carrier frequency so that the modulator frequencies trailed other side band being amplified by the power amplifier i4.

The system of Fig. 2 is very similar to that of Fig. 1, except that herein a steel tape is used instead of a paper or metal foil tape, and phase or frequency modulation is employed by means of suitable well known apparatus, herein conventionally indicated by box I6.. It is preferred to employ the transmitter system of this figure instead of that of Fig. 1, because of the greater amount of power which may be obtained over Y each channel.

and one of the side bands are eliminated, the

For receiving the signals sent out over the system of Fig. 1, any suitable arrangement may be employed wherein a carrier wave is reintroduced at the receiver and the audio frequencies characteristic of the various channels filtered out to record the signals on a suitable scanning head which preferably is identical with that used on the transmitter. One suitable way of accomplishing this is disclosed in my United States Patent No. 1,751,584, to which attention is invited.

What is claimed is:

1. A multiplex facsimile system comprising a plurality of mechanically scanning elements contacting a moving tape having electrically .conducting characters thereon, said elements being arranged substantially in a straight line at a slant with respect to the direction of travel of the tape, a plurality of sources of oscillations each having a different frequency and being in circuit with a different scanning element, means for continually changing the degree of slant of said scanning elements, whereby secrecy of signal transmission is insured, and means controlled by the simultaneous closure of several of said scanning elements upon an electrically conducting character for transmitting a complex wave comprised of the different frequencies in circuit with said closed scanning elements.

2. In a multiplex facsimile system, the combination with a source of oscillations of different frequencies, of a plurality of filters for separating the oscillations of different frequencies, and a plurality of scanning contacts each being connected to one of said filters for transmitting a characteristic frequency, said contacts being arranged at a slant with respect to the direction of travel of a tape having signal marks thereon. means for insuring secrecy in the transmission of signals comprising a cam for continually changing the degree of slant of said contacts, and means for simultaneously closing the circuit of one or more of said contacts in accordance with the configuration of the signal mark to be transmitted, whereby there is obtained a complex wave comprised of .the various frequencies transmitted by said simultaneously closed contacts.

3. In a secret multiplex facsimile system, the

combination with a source of oscillations of different frequencies, of a plurality of filters for separating the oscillations of different frequencies, and a plurality of scanning contacts each being connected to one of said filters for transmitting a characteristic frequency, a feeder line,

and means for associating one or more of said contacts with said line in accordance with the configuration of the signal impressed on a moving tape, said contacts being arranged at a slant with respect to the direction of travel of said tape, whereby there' is transmitted over said line a complex wave comprised of the various frequencies transmitted by those contacts which are simultaneously associated with said line, cam mechanism for varying the relative positions of the scanning contacts, and a radio transmitter in circuit with said line including a source of carrier oscillations and means for suppressing the carrier and transmitting the side bands over the ether.

4. In a multiplex system having a plurality of scanning elements, and a plurality of oscillations each having a different frequency and being in circuit with a different scanning element, and means controlled by the simultaneous circuit closure of several of said scanning elements for transmitting a complex wave comprised of the different frequencies in circuit with said closed scanning elements, the method of increasing the power and effectiveness of the transmitted multiplexed signals which includes the step of adjusting the relative phase, timing and diversity of the component signal currents so that their combined peak value is reduced in proportion to the strength of each current.

5. In a secret multiplex facsimile system, a scanning head comprising a tape having a non-conductive layer with electrically conducting message symbols, and a plurality of pairs of sending contacts, each pair being individual to a channel, a source of oscillations associated with each channel, said channels each having a Wave of characteristic frequency, a transmission medium, a simultaneous closure of contacts of different channels on said conductive message symbols being arranged to cause the transmission over said medium of a complex Wave composed of the individual frequencies characteristic of the channels whose contacts are closed, and means for producing relative motion between said tape and said contacts, said contacts being arranged along a line making an acute angle with respect to the direction of travel of said tape.

6. In a multiplex facsimile system, a scanning head having a non-conductive layer with electrically conducting message symbols, and a plurality of sending contacts, there being at least one contact individual to a channel, a source of oscillations associated with each channel, said channels each having a wave of characteristic frequency, a transmission medium, a simultaneous closure of contacts of different channels on said conductive message symbols being arranged to cause the transmission over said rnedium of a complex wave composed of the individual frequencies characteristic of the channels whose contacts are closed, and means for producing relative motion between said layer and said contacts, said contacts being arranged along a line making an acute angle with respect to the direction of travel of said layer.

'1. In a multiplex facsimile system, a scanning head comprising a tape carrying electrically conducting message symbols, and a plurality of sending contacts, there being at least one contact individual to a channel, a source of oscillations associated with each channel, said channels each having a wave of characteristic frequency, a transmission medium, a simultaneous closure of contacts of different channels on said conductive message symbols being arranged to cause the transmission over said medium of a coniplex wave composed of the individual frequencies characteristic of the channels whose contacts are closed, and means for producing relative motion between said tape and said contacts, said contacts being arranged along a line making an acute angle with respect to the direction of said tape.

8. In a multiplex facsimile system, a scanning head comprising a tape carrying electrically conducting message symbols, and a plurality of sending contacts, there being at least one contact individual to a channel, a source of oscillations associated with each channel, said channels each having a wave of characteristic frequency. a transmission medium, a simultaneous closure of contacts of different channels on said message symbols being arranged to cause the transmission over said medium of a wave whose length at any instant is controlled by a complex Wave composed of the individual frequencies characteristic of the channels Whose contacts are closed, and means for producing relative motion between said tape and said contacts.

9. In a multiplex facsimile system, a scanning head comprising a tape carrying electrically conducting message symbols, and a plurality of sending contacts, there being at least one contact individual to a channel, a source of oscillations associated with each channel, said channels each having a wave of characteristic frequency, a transmission medium, a simultaneous closure of contacts of different channels on said message symbols being arranged to cause the transmission over said medium of a wave whose length at any instant is controlled by a complex wave composed of the individual frequencies characterstic of the channels Whose contacts are closed, and means for producing relative motion between said tape and said contacts, said contacts being arranged at a slant relative to the direction of travel of said tape.

10. In a multiplex facsimile system, a. scanning head comprising a tape carrying message symbols, and a plurality of scanning elements, there being a scanning element for each channel, a source of oscillations associated with each channel, said channels each having a wave of characteristic frequency, a transmission medium, the simultaneous circuit closure of scanning elements of diierent channels caused by the con figuration of any message symbol being arranged to cause the transmission over said medium of a wave whose length at any instant is controlled by a complex wave composed of the individual frequencies characteristic of the channels whose scanning element circuits are closed, and means for producing relative motion between said tape and said scanning elements.

11. In a multiplex facsimile system, a scanning head comprising a tape carrying message symbols, and a plurality of scanning elements, there being a scanning element for each channel, a source of oscillations associated with each of travel channel, said channels each having a Wave of CLARENCE W. HANSELL. 

